1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to a switching filter module for selectively receiving wireless communication channels having good reception sensitivity simultaneously among a plurality of wireless communication channels.
2. Description of the Related Art
Unlike cellular communication schemes for providing high-speed mobile service, WLAN (Wireless Local Area Network) have been adapted to satisfy the demand for high-speed data service from users stationed at relatively fixed locations, and has recently boosted its market growth. In accordance with the international WLAN standard commonly accepted so far, IEEE 802.11a/b/g, a bandwidth of approximately 20 MHz is used for each channel. For providing of higher throughput multimedia service, however, there is a need for introducing a system using a wider bandwidth. This wideband WLAN system uses one or more channels based on the current standard in combination, for compatibility with existing WLAN systems.
Implementation of a module that can select a plurality of channels simultaneously requires a plurality of filters, each for selecting a signal frequency corresponding to a channel.
FIG. 1 illustrates the configuration of a conventional switching filter module for simultaneously selecting one or two channels from among four available channels. While the filter module is shown to be applied to a low-band receiver in a WLAN system, it can also be applied to a transmitter.
Referring to FIG. 1, a reception antenna 102 receives an RF (Radio Frequency) signal. A band pass filter (BPF) 104 filters the RF signal and outputs a filtered signal in a relatively wide frequency band. A low noise amplifier (LNA) 106 amplifies the filtered signal. A mixer 108 downconverts the frequency of the amplified signal to an IF (Intermediate Frequency) signal using a local oscillation frequency generated from a local oscillator 110. Since the IF signal contains a plurality of channel signals in predetermined frequency bands, a switching filter module 112 selects desired channel signals for use and feeds the selected signals to a demodulator (not shown).
The switching filter module 112 comprises channel selection filters 116a to 116g for passing one or two desired channel signals, while suppressing the other channel signals contained in the IF signal. For ease of reference, a number (or numbers) is provided on each channel selection filter that represents a channel that the filter passes. An on-off switch (one of 114a to 114g) is provided at the front end of each channel selection filter.
The switching filter module 112 processes the four channel signals having successive adjacent frequency bandwidths. In the case where channels #1 and #4 are selected at the same time, the switching filter module 112 turns on the first and fourth switches 114a and 114d at the front ends of the first and fourth channel selection filters 116a and 116d, while turning off the switches 114b, 114c, 114e, 114f and 114g at the front ends of the other channel selection filters 116b, 116c, 116e, 116f and 116g. If adjacent channels #1 and #2 are to be selected simultaneously, the switching filter module 112 turns on the first and second switches 114a and 114b at the front ends of the first and second channel selection filters 116a and 116b, while turning off the other switches 114c to 114g. 
The switching filter module 112 can select two adjacent channels by use of two of four channel selection filters each of which can select only one channel. In this case, however, since the two independent channel selection filters are used in parallel, each of both filters suppresses its adjacent channel signal. For example, when the switching filter module 112 selects two adjacent channels at the same time according to IEEE 802.11a WLAN using an effective bandwidth of 16.6 MHz for each channel, an effective bandwidth of only 33.2 MHz (=16.6×2) is available, as illustrated in FIG. 2A.
On the other hand, if two adjacent channels are selected using the channel selection filters 116e to 116g can select the entire bandwidth of two channels from the start frequency of a lower channel to the end frequency of a higher channel, an effective bandwidth of 36.6 MHz can be obtained, as illustrated in FIG. 2B. A maximum data rate gain of the 802.11a scheme, which can be achieved as the effective bandwidth increases, is up to about 11%.
As described above, the concurrent selection of two adjacent channels by use of two of the four available channel selection filters of which each selects only one channel reduces efficiency of frequency usage. To overcome this problem, the channel selection filters 116e, 116f and 116g are additionally needed to simultaneously select two adjacent channels. Consequently, the switching filter module 112 requires relatively many, i.e., seven channel selection filters to select one or two channels from four channels. Similarly, 15 channel selection filters are needed for selection of 8 channels, and 31 channel selection filters for selection of 12 channels.
In the above conventional switching filter module, (2N−1) channel selection filters are required for N available channels. Therefore, as the number of available channels increases, the number of filters required to achieve desired response characteristics is significantly increased. As a result, the size and cost of the module are increased.